Tea and coffee may wake you up and keep you focused, but don’t overdo it on the caffeine — it may trigger migraines or IBS in people who are sensitive.

Caffeine is a natural chemical that activates the central nervous system, which means that it revs up nerves and thought processes. Regular caffeine consumption, from coffee and/or tea, has been shown to increase short-term focus and alertness, as well as long-term memory. Although most people enjoy caffeine’s “revved up” effect, some people are caffeine-sensitive and are left feeling jittery or ill after ingesting a dose. If you fall into the second group, you’ll want to eliminate caffeinated beverages or adjust your intake to match your personal tolerance. Those with sleeping problems or insomnia may need to stop drinking caffeinated beverages up to eight hours before bedtime (or omit entirely).

Additionally, caffeinated beverages can sometimes trigger migraine headaches in people who are sensitive. And IBS sufferers take note: Some people with IBS become symptomatic after ingesting caffeinated coffee or tea.

Caffeine may also have some adverse effects on women just before their menstrual cycles. Some research suggests that the effects of caffeine become magnified for women when they are premenstrual. Caffeine may exacerbate PMS symptoms and cause greater breast tenderness, nervousness, and irritability. If this is true for you, switch to herbal teas or decaffeinated beverages at this time in your cycle.